Light-emitting diodes (LEDs) are good candidates to replace incandescent and other light sources. LEDs have higher power to light conversion efficiencies than incandescent lamps and longer lifetimes. In addition, LEDs operate at relatively low voltages, and hence, are better adapted for use in many battery-powered devices. Furthermore, LEDs are a better approximation to point sources than a fluorescent source, and hence, are better adapted than fluorescent sources for lighting systems in which a point light source that is collimated or focused by an optical system is required.
An LED can be viewed as a three-layer structure in which an active layer is sandwiched between p-type and n-type layers. Holes and electrons from the outer layers recombine in the active layer to produce light. Part of this light exits through the upper horizontal surface of the layered structure. Unfortunately, the materials from which the outer layers are constructed have relatively high indices of refraction compared to air or the plastic encapsulants used to protect the LEDs. As a result, a considerable portion of the light is trapped within the LED due to internal reflection between the outer boundaries of the LED. This light exits the LED through the side surfaces. To capture this light, the LEDs are often mounted in a reflecting cup whose sidewalls redirect the light from the sides of the LED into the forward direction. In addition, the cups are often filled with a clear encapsulant that protects the LED die and can provide additional optical functions such as having a surface that is molded to form a lens.
Unfortunately, the packages must be able to withstand relatively high processing temperatures. AuSn eutectic die attachment can subject the package to temperatures as high as 320 degrees centigrade. In addition, LEDs designed for high power applications generate significant amounts of heat that result in further temperature cycling of the package when the LEDs are turned on and off. As noted above, the cups are typically filled with an encapsulant. The encapsulant material is different from the material from which the reflector is formed. As a result, the encapsulant material and the material from which the reflector is formed typically have different coefficients of thermal expansion. In addition, the adhesion of the encapsulant to the reflector is often less than ideal. As a result, the encapsulant tends to delaminate from the cup after multiple thermal expansion cycles.